The present invention relates generally to noise attenuators and more particularly to improvements in exhaust gas mufflers of the type having an enclosure with an inlet for receiving exhaust gas from an engine and an outlet for venting the exhaust gas to the atmosphere.
Internal combustion engines having exhaust gases ported directly to the atmosphere emit loud and noxious noises with this problem being accentuated somewhat in two stroke cycle engines and hence the need for some type exhaust noise attentuation has long been recognized. Attempts to fill this need have generally taken the form of an enclosure with inlet and outlet openings containing one or more perforated baffles.
Typically the muffler shell or housing is formed with a generally tubular shape of circular or oval cross section and opposed generally flat ends through one of which an inlet pipe passes and through the other of which an outlet pipe passes. Some muffler housing designs have had their shape dictated by rather stringent application limitations and have been formed as stamped sheet metal portions joined together and generally having at least a pair of flat sides. This latter type design might for example be required in a chain saw muffler. At least one spherical muffler housing has also been proposed.
The flat sided muffler housings as well as the tubular housings both have relatively large flat surfaces which are unfortunately good sound radiators tending to defeat the noise attenuating purpose of these mufflers. The proposed spherical muffler housing obviates this problem but unfortunately accentuates a different problem. The engine output consumed in pumping the exhaust gas through the muffler system cannot be harnessed as useful output and thus the harder the engine must work to exhaust the gases the less efficient that engine will be. So-called back pressure, that is, gas pressure opposing the exhausting operation increases as a muffler volume decreases and, since the space available as determined by the dimensional constraints of the engine environment is rarely spherical, the spherical design muffler typically does not utilize nearly all of the space available and therefore presents a characteristically small volume and therefor relatively higher back pressure reducing engine efficiency. The size and number of openings in perforated baffles within a muffler represents a similar trade off wherein enhanced sound attenuation results in reduced engine efficiency.